Straightness has been measured by arranging an optical system so that deviation of the test beam transversely of its path as an element moves along a line also changes the path length of the test beam relative to the reference beam to produce interference fringes that allow measurement of the transverse deviation. The prior art has accomplished this with a movable birefringent element such as a Wallaston prism that travels along the line and splits a beam into two beams at an acute angle to each other, forming a Y-shape. Both of the angled beams are reflected back on themselves from accurately perpendicular mirrors; and if the prism deviates transversely from the beam path, it changes the path lengths of the two beams to produce interference fringes and a measurement of the transverse deviation. Such an arrangement can measure the straightness of the bed of a lathe, machine tool, or other line along which the prism can move.
The prior art system suffers many disadvantages. It requires large, expensive mirrors with accurately flat surfaces set accurately perpendicular with the beams. The working length of prism travel is limited by the mirrors so that the instrument has to be made in several sizes for measuring different lengths. Most serious of all, however, is that the small acute angle between the beams diverged by the birefringent prism optically reduces the error signal by a factor of 36 in translating transverse deviation into an optical path difference. This requires expensive detector electronics to amplify and process the optically reduced error signal so that the electronics can make up for the loss and produce a sufficiently accurate signal. All these disadvantages cooperate to make prior art straightness measuring instruments expensive, temperamental, costly to maintain, and limited in scope and accuracy.
I have devised a better way of producing an optical path difference between a reference and test beam in response to transverse deviation relative to the test beam. My instrument not only eliminates optical reduction of the transverse error, but can optically magnify this error to increase the accuracy and reduce the burden on the detector electronics. My instrument can also measure over both short and long range distances without modification; and besides being usable for straightness measurement, it can measure angular deviation and small deviations transverse to the test beam by any one of several alternatives. These features make it less expensive, more accurate and reliable, easier to operate and maintain, more versatile, and usable over wider distance ranges.